here is how my mind set is with a prequel. there are only 2 reasons to make a prequel 1. to expand on the lore of your universe or 2 because you have no way of going forward beacuse of the ending of the original game,movie, what have you.

I think it would be a prequel in case of players who have never played the original Borderland games before this. If it's a sequel though, telltale would have to write a story that not only provides a background of the setting, but not outright spoil the events of previous games.

I just read, it takes place after the final DLC of Borderlands 2. Sequel.
As to Jack, I'm wondering if he somehow got into the Digistruct system before his demise in the Warrior's Vault. In the trailer, his appearance reminded me of Digistruct Peak.